[GHC] #11572: BangPatterns-related behavior regressions on GHC 8.0

[GHC]

#11572: BangPatterns-related behavior regressions on GHC 8.0
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  8.0.1-rc2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 When you compile this code:

 {{{#!hs
 {-# LANGUAGE BangPatterns #-}

 main :: IO ()
 main = let !_ = (undefined :: ()) in print 2
 }}}

 with GHC 8.0.1-rc2 with any level of optimizations, it will print `2`
 instead of throwing `undefined` like GHC 7.10.3 does.

 Note that changing `undefined :: ()` to something like `undefined :: Int#`
 is enough to invoke `undefined` when compiled, but if you run this code in
 GHCi:

 {{{
 λ> :set -XMagicHash -XBangPatterns
 λ> import GHC.Prim
 λ> let !_ = (undefined :: Int#) in print 2
 }}}

 You get something even stranger:

 {{{
 ghc: panic! (the 'impossible' happened)
   (GHC version 8.0.0.20160204 for x86_64-unknown-linux):
         dsLet: unlifted
   !_ = (undefined :: Int#)
   print @ Integer $dShow_a2ky 2

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11572>
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